ABSTRACT
Some of the alternative designs of offshore structures that are possible in the ocean environment are given in Figure 7.1 [1]. These offshore structures can be broadly classified as (i) bottom-fixed/ founded structures, (ii) bottom-supported compliant structures, and (iii) compliant floating structures. Bottom-fixed structures consist of fixed steel jacket platforms, concrete gravity platforms, and the hybrid (concrete base with steel jacket supporting the top steel deck structures). They are comparatively stiff structures embedded into/supported on relatively soft ocean sediments. The framed steel jacket structure is supported on a number of deep-driven main/skirt pile clusters, with mud mats, as shown in Figure 7.2 [2]. The gravity structure is supported on a large concrete gravity base, with skirts (located below the concrete base) penetrating deep into the soft top soil. Under the wave excitation, the structures exhibit high elastic restoring forces, and as such, the structural displacements are considered to be small. In the case of bottom-supported compliant structures, the relative vertical motion between the well heads (at bottom) and the deck should be minimized. Moreover, if subsea (seabed) completions are used as in the case of the floating compliant platform shown in Figure 7.1, the connection to the seabed can be made with the less-costly flexible risers and control lines; in this case, the structural motions (rather than displacements) will be much higher.
